How to access the secret exit in Doom II: No Rest for the
Living level 4, Hell Mountain, which leads to level 9,
March of the Demons:

(Note: To view a video demonstration of reaching the secret
exit on this level, click here.)

When you first appear on this level, you'll be at the south
end of the map and facing east, down a long, dark corridor.
Go to the end of the corridor and turn left/north.  Proceed
into the outdoor area, and continue north, staying close to
the right-hand wall as it curves around to the west.
Continue north/northwest, staying toward the right when
possible, until you come to the end of the open area (which
is near the center of the level).

Nearby you may see a curving, stair-stepped tunnel leading
clockwise up and around to the north, with blue health
potions sitting along the left-hand wall on the steps.
Just southwest of that curving tunnel's entrance you'll see
a demon-faced wall panel.  Walk up to it, and it will drop
down, allowing access to the west half of the level.

Step through, and look right/north for a closed wooden
door, and go through it to enter the east side of a large
chamber.  Walk north a few paces, turn left and head west.
Continue west to a 3-way intersection and turn left/south.
Go south through the wooden door into a smaller chamber,
and continue south through the next wooden door.
Go up the steps and go through the door on your left at the
top of the steps.

Walk a few paces forward/east, until you come to a red
pool.  Look right/south, and you'll see a wide, red
waterfall.  Step through the waterfall into a small, dark
area beyond.

Activate the back/south wall of this place, and it will
drop down to reveal a teleporter pad.  Step on, and you'll
appear in a circular chamber located at the north end of
the level.
Pick up the red skull key here, and step on the nearby
teleporter pad.

You'll re-appear back at the red pool, standing just north
of the waterfall, and facing north.
Follow the red pool north, dropping down a level.  Continue
north a few paces until you come to a fountain-like object
in the pool.  Turn right/east, and look for shallow steps
leading up and northeast to where the yellow key sits on
the ground.

Grab it and head north a few paces to the wooden door, and
go through.  (This is the same wooden door you went through
just after lowering the demon-faced wall panel earlier.)
As you enter the chamber continue straight ahead/north
this time, to the yellow-keyed door ahead.  Go through it
to enter a short north-south chamber.

Go north to the far end and out the open doorway there.
Continue north a few paces to a small brown rocky structure
ahead.  On the southeast/right side of this structure is a
short wooden elevator/lift.  Activate it and ride up.

At the top of the structure, on its west edge, is a switch.
Beyond the switch you'll see the open end of a short
east-west passageway, ahead and below you, where green and
brown walls meet.  Activate the switch, and the right-hand
side of that short passageway will open up to form a new
passage leading north.
From the switch, drop down and go into that newly opened
northern passage.

Along the floor on either side of this short, dark passage
you'll see candles.  Follow the right-hand wall a few paces
until you come to a shallow indentation in the wall.
Activate this wall panel like a switch, and it opens to
reveal a tiny chamber, with a skull switch in the chamber's
north wall.  (Note your position here in this tiny chamber,
as you will come back to it in a moment.)

Activate that skull switch, step back out of the chamber,
and look right/north to see that another area has now
opened up.

Walk north a couple of paces and look to see 3 vertical
bars blocking off the base of a stairway on the left, and
a second skull switch to the right, on the east wall.

Activate this switch, then go back a few paces south, then
east to the first skull switch in the tiny chamber, and
you'll see that the east wall of the chamber has opened up
to reveal a few small steps that lead up to the east.

Go up the steps, and at the top look left for a third skull
switch, and activate it.
Head back down the steps, past the first skull switch, and
turn right/north and head to the stairway that was blocked
by the vertical bars (now removed).

Go west up the stairs.  Note that halfway up this staircase
there is an invisible floor trigger (or "tripwire"), which
temporarily opens the secret-exit chamber.

As you get to the top of the stairs, a short elevator/lift
will drop down momentarily, then rise again.  Get on, ride
up, then quickly Run forward/west to the secret-exit
chamber.  It will be ahead and slightly to the left/south.
If the chamber closed before you got inside, simply go back
to the stairway and try again.

Click here to access a YouTube video which demonstrates the
walkthrough above.